KVM: e500: perform hugepage check after looking up the PFN

e500 KVM tries to bypass __kvm_faultin_pfn() in order to map VM_PFNMAP
VMAs as huge pages.  This is a Bad Idea because VM_PFNMAP VMAs could
become noncontiguous as a result of callsto remap_pfn_range().

Instead, use the already existing host PTE lookup to retrieve a
valid host-side mapping level after __kvm_faultin_pfn() has
returned.  Then find the largest size that will satisfy the
guest's request while staying within a single host PTE.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>